This application includes subject-matter incorporated from applicant""s British Patent Application Ser. No. 9814121.1 filed on Jul. 1, 1998.
The invention relates to a stratum such as a separator plate for use as a subcomponent of a fuel cell stack, and especially a PEM-type fuel cell stack (xe2x80x9cPEMxe2x80x9d is an acronym for xe2x80x9cproton exchange membranexe2x80x9d), and to preferred methods of use of such strata in a fuel cell stack.
Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. In electrochemical fuel cells employing hydrogen as the fuel and oxygen as the oxidant, the reaction product is water. Solid polymer fuel cells generally include a membrane electrode assembly, (xe2x80x9cMEAxe2x80x9d) comprising a solid polymer electrolyte or ion exchange membrane disposed between two electrode layers. The electrode layers typically comprise porous, electrically conductive sheet material and an electrocatalyst at each membrane-electrode interface to promote the desired electrochemical reactions.
At the fuel cell anode, the fuel (typically hydrogen) moves through the porous electrode material and is oxidized at the anode electrocatalyst to form cations, which migrate through the membrane to the cathode. At the cathode, the oxidizing gas (typically air containing oxygen) moves through the porous electrode material and is reduced at the cathode electrocatalyst to form a reaction product, usually water. The anode and cathode also respectively donate and accept the electrons required for the electric current flow from and to the fuel cell and ultimately through the load across which the fuel cell alone or in electrical combination with other fuel cells (usually a series connection in a stack) is connected. More specifically, in the vicinity of the anode of each fuel cell in the stack, the hydrogen breaks down into (i) the positively charged protons that move through the polymeric membrane, and (ii) electrons that flow to the next fuel cell connected in series in the stack (or, if the fuel cell is a terminating fuel cell in a stack, through the load across which the fuel cell stack end terminals are connected). In the vicinity of the cathode, not only do the fuel and oxidant gases complete the exothermic chemical reaction that provides the water or other reaction product, but the electric circuit is also completed; the electrons that flow through the load combine with the cations and the oxidant to form an electrically neutral reaction product, usually water. The hydrogen may be supplied directly from a supply of same or may be a conversion product of, for example, a hydrocarbon such as methane.
In conventional fuel cells, the MEA is interposed between two substantially fluid-impermeable, electrically conductive plates, commonly referred to as separator plates. The plates serve as current collectors, provide structural support for the electrode layers, typically provide means for directing the fuel and oxidant to the anode and cathode layers, respectively, and typically provide means for removing products, such as water, formed during operation of the fuel cell. When reactant channels are formed in the separator plates, the plates are sometimes referred to as fluid flow field plates.
Fuel cell stacks typically comprise an aligned assembly of fuel cells connected together mechanically and electrically in series to obtain desired voltage and power output. An early example of a fuel cell stack is illustrated in Maru U.S. Pat. No. 4,444,851 granted Apr. 24, 1984; a later example is illustrated in Washington U.S. Pat. No. 5,514,487 granted May 7, 1996. Typically, in any such stack, one side of a given fluid flow field plate (separator plate) is the anode plate for one cell, and the other side of the plate is the cathode plate for the adjacent cell, and so on seriatim. For this reason, the plates are sometimes referred to as bipolar plates.
Fluid reactant streams are typically supplied to the fuel cell electrodes via channels in the flow field plates communicating with external plenum chambers or manifolds connected to the sides of the stack, or communicating with internal plenum chambers or manifolds formed by aligning openings formed within the plates and MEAs in the stack. Internal manifolds have been almost universally used in preference to external manifolds for proton exchange membrane (PEM) fuel cell stacks; external manifolds are more commonly found in high-temperature fuel cell stacks such as phosphoric acid fuel cell stacks. Similarly, fluid stream exhaust conduits or manifolds may be external or internal. Typically the stack also has coolant passageways extending within it for circulating a coolant fluid to absorb heat generated by the exothermic fuel cell reaction.
The requisite flow-field channels in a fuel cell separator plate may be formed as a pattern of parallel open-faced fluid-flow channels formed in a major surface of a rigid, electrically conductive plate. The parallel channels extend between an inlet manifold opening and an outlet manifold opening formed in the plate. Watkins U.S. Pat. Nos. 4,988,583 and 5,108,849 issued Jan. 29, 1991 and Apr. 28, 1992, respectively, describe fluid-flow-field plates in which continuous open-faced fluid-flow channels formed in the surface of the plate traverse the central area of the plate surface in a plurality of passes, that is, in a serpentine manner, between an inlet manifold opening and an outlet manifold opening formed in the plate. Fluid-flow-field plates for electrochemical fuel cells in which the inlet and outlet flow channels are mutually disconnected, so that in operation the reactant stream must pass through the porous electrode layer to get from the inlet to the outlet, have also been described.
In a companion British application Serial No. 9814123.7 (McLean et al., assigned to the applicant herein) filed on Jul. 1, 1998 there is disclosed in one embodiment of the invention of that application an undulate MEA layer sandwiched between successive planar separator plates for use in a PEM-type fuel cell stack. (xe2x80x9cMEAxe2x80x9d is an acronym for xe2x80x9cmembrane electrode assemblyxe2x80x9d.) In conjunction with the separator plates, the undulations of the MEA layer serve to separate fuel flow channels from oxidant flow channels. Those flow channels bounded on one side by one separator plate of the sandwich are the fuel flow channels, and those channels bounded by the other of the two separator plates in the sandwich are the oxidant flow channels. In such undulate MEA layer/separator plate sandwiches, typically a given separator plate serves as the lower layer of a sandwich for one fuel cell and as the upper layer of the sandwich for the fuel cell immediately underneath it (assuming for the sake of this discussion, that the fuel cell stack is vertically oriented with the fuel cells forming generally horizontal strata in such stackxe2x80x94of course, the orientation is arbitrary). Consequently, for any given separator plate, fuel gas flows in a flow field on one side of the plate, and oxidant gas flows in a flow field on the other side of the plate. In each case, the flowpaths of the flow field are defined by the spaces formed by the undulations of the MEA layer. These flow channels extend in an axial sense from one side of the fuel cell stack to the opposite side.
In many conventional configurations, the flow channels form a serpentine flowpath. The term xe2x80x9cserpentinexe2x80x9d is understood to apply to a flowpath in which, in sequential flow channel components of the flowpath, a reversal of direction of gas flow occurs. Such flowpaths serve two principal objectives, viz to provide reactant gas efficiently to as much of the MEA layer surface as possible, and to provide flow channels that are long relative to their cross-sectional area.
If a serpentine flowpath design is chosen, it is often conveniently formed for the most part of straight channel segments that extend over most of one dimension of the working fuel cell surface and connect at their extremities to the next adjacent straight segment forming part of that particular flowpath. Representative such flowpath designs are illustrated, for example, in Washington U.S. Pat. No. 5,300,370 issued Apr. 5, 1994. Flowpath configurations other than serpentine may also conveniently include a number of relatively long straight channel segments requiring connection at their ends. Whatever the flowpath configuration, if it comprises through much of its length a series of parallel straight channel segments, then unless the channel segments are part of a continuous integral whole flowpath, some means at each end of each of the flow channels must be devised to interconnect selected channels so as to provide the requisite serpentine or other suitable flowpath configuration.
Further, it is not unusual in conventional fuel cells to provide in a reactant flow field a number of different flowpaths winding their way across the fuel cell surface in generally parallel alignment. The separation of such flowpaths normally need not be absolute; small imperfections and leaks do not normally deleteriously affect fuel cell performance; further, occasional concurrence of two or more flowpaths in a flow field may be advantageous for pressure and reactant concentration equalization, or for other purposes. It is, of course, necessary to avoid serious leaks that would permit a significant quantity of reactant gas to flow from an inlet point to an outlet point of a flow field by a short circuit (lower pressure drop path); it is necessary to preserve the basic integrity of the serpentine or other designed flowpath arrangement.
It will be understood that various means can be devised for interconnecting flow channels to provide the requisite serpentine or other selected flowpath from a selected inlet to a selected outlet in the flowfield. Various end coupling arrangements can be devised at the ends of the axially extending flow channels formed by the undulate MEA layer in conjunction with the separator plates between which it is sandwiched, in order to achieve this objective. However, some such coupling and interconnection arrangements can be relatively expensive to manufacture, and may require accurate alignment with the flow channels to be effective. For example, an end manifold for interconnecting flow channels in a fuel cell stack comprising, say, 100 fuel cells would have to provide end connections for thousands of channelsxe2x80x94a formidable manufacturing and alignment task.
Alsthom-Atlantique U.K. patent specification GB 2 158 989 published Nov. 20, 1985 illustrates in FIGS. 10 and 11 thereof a display bipolar separator plates (referred to as bipolar current collectors), with a number of grooves or hollows in the planar faces of each plate that constitute flow channels for fuel and oxidant gas, respectively, and that run over the central portion of the plate in a direction parallel to the longitudinal axis of the plate. The network of channels can be formed by corrugation of the central portion of the plate. The gas is supplied to the plates from either an internal or external manifold to a series of channels that fan out from the manifold inlet to the central portion of the separator plate. Identical channels on the opposite end of the plate serve to remove exhaust gas. The flow channels in contact with the active region of the MEA are parallel to each other, and the ends of the channels opens into a common space in communication with the supply and exhaust channels, respectively. As a result, gas is supplied to a series of parallel flow channels by way of a common inlet and outlet opening. Thus, gas flows in a single direction across the surface of the plate.
Alsthom-Atlantique nowhere suggests that its flowpath configuration could be modified to provide a serpentine flowpath arrangement. Nor does Alsthom-Atlantique disclose a structure that includes or that could be readily modified to accommodate undulate configurations of the type disclosed in Applicant""s British application Serial No. 9814123.7.
The present inventors have devised a very simple expedient for interconnecting, or coupling, flow channels together to form flowpaths of a flow field within a fuel cell sandwich (by xe2x80x9csandwichxe2x80x9d is meant a repeating subassembly within a fuel cell stack). The sandwich preferably comprises an undulate MEA and adjacent separator plate together constituting a subcomponent of a PEM-type fuel cell stack of the type described above. Interconnection of the flow channels into a suitable flowpath, preferably a serpentine flowpath comprised of flow channels interconnected so that reactant gas flows in alternating directions in sequentially connected channels, is provided within such fuel cells by providing an array of interconnection couplings in a stratum (a stratum typically being either a separator plate or an MEA layer, or some combination of layers or portions of layers including a separator plate or an MEA layer ). These couplings link together the flow channels into a pattern of suitably interconnected flow channels. The flow channels may conveniently be depressed cavities in the separator plates.
A similar arrangement of couplings may be used to couple together the parallel flowpaths of flowfields in a fuel cell stack in which undulate layers are present; in such case, while a suitable pattern of serpentine flowpaths remains a preferred objective, the inventors are not aware of any such interconnection arrangement that has previously been used to interconnect flow channels in association with undulate layers of any sort, whether or not serpentine flowpaths are formed. The couplings in such fuel cell stacks may conveniently be interconnection conduits or gaps formed by flattening or truncating small portions of the apices (or nadirs) of undulate MEA layers.
It is of course an objective that the structure providing the fuel gas set of couplings does not interfere with the structure providing the oxidant gas set of couplings, nor vice versa. Accordingly, at each longitudinal end of the fuel cell sandwich, the couplings for the oxidant gas flow channels are preferably transversely offset from those for the fuel gas flow channels.
The interconnecting couplings may be formed by stamping or moulding the stratum, for example. While the interconnection couplings may be formed in either the separator plate or MEA layer in a PEM-type fuel cell, by way of example and for simplification, much of the following discussion will refer to the formation of such couplings as cavities in the separator plate. It should be understood when reading the description that the designer could choose to form the couplings in the MEA layer instead. It should be further understood that while for simplicity the present invention is easiest presented on the assumption that at least one of the layers in the fuel cell sandwich is planar, either or both the MEA layer and separator layer may be non-planar.
The principles of flow channel interconnection as described herein using PEM-type fuel cells by way of example may be used in the design of other types of fuel cell.
Referring by way of example to flow channel couplings in separator plates, it is noted that in a thin separator plate, depressions on one major surface of the plate appear as embossments on the other, and vice versa. The depressions can be formed by stamping the separator plate, or by molding the separator plate to provide the requisite depressions. It is necessary that the depressions be hollowed out sufficiently to provide cavities of adequate size into and out of which reactant gas may flow for the purpose of interconnecting adjacent reactant gas flow channels. Suitable easily manufactured interconnection couplings providing a short flow conduit between adjacent flow channels are of the essence of the invention; the embossments are not of the essence, and in a thick enough plate, the depressions would not necessarily appear on the opposite sides of the plate as embossments. However, economy of materials and manufacture militate in favour of thin separator plates in which the depressions necessarily appear on the other side of the plates as embossments.
In this specification and the appended claims, the three mutually perpendicular directions and dimensions of interest are for convenience of description and definition named as follows:
(a) The stack dimension is the dimension extending from one extremity of the fuel cell stack to the other, perpendicular to the layers of the stack and perpendicular to the broad working surfaces of the MEA strata and to the broad surfaces of the separator plate herein described.
(b) The longitudinal dimension is the dimension parallel to the interconnected flow channels on the separator plate herein described.
(c) The transverse dimension is the dimension perpendicular to the other two dimensions.
Assuming that the designer chooses a serpentine flowpath (or an array of serpentine flowpaths) or some other suitable configuration in which parallel long straight channels are interconnected, the depressions and embossments are conveniently formed at or near the ends of the separator plate (in an axial sense), so that they may interconnect selected ones of the long straight flow channels formed in the undulate MEA/separator sandwich to form one or more serpentine flowpaths (say). On a given surface of the separator plate, it is apparent that only depressions will be effective to interconnect adjacent flow channels; embossments do not. The depressions must have a width in the transverse sense (i. e., perpendicular to the axial or flow sense) that is sufficient to enable adjacent flowpaths to be adequately interconnected, but the boundaries of the depressions should not come unduly close to the nearest MEA surfaces. Equally, the depth of the depressions must be sufficient to enable reactant gas to flow freely from one flow channel via the depression to the next adjacent flow channel. Accordingly, the width of a depression cannot be appreciably wider than about half the distance between adjacent apices of the undulate MEA layer, because otherwise, the depression would tend to interfere with the access of reactant gas on the other side of the separator plate to the next consecutive MEA layer in the stack. So the shape and size of the depression must be designed both with its interconnectivity objective in mind, and also with the objective of avoiding interference with the flow of reactant gas to the MEA layer on the other side of the separator plate in mind (with the understanding that a depression on one surface of the separator plate may appear on the opposite surface as an embossment that will partially penetrate into the flow channel on the other side of the separator plate).
Note that depending upon the flow field design, more than two channels in a group of sequential flow channels adjacent a stratum may be coupled together at one longitudinal end thereof; in such cases, it is necessary to consider the implications of such interconnection not only for the flow field directly affected but also for the flow field on the other side of the stratum, since in the absence of proper design, there may be competing demands from the flow fields on either side of the stratum for overlapping interconnection space. If the couplings for the oxidant gas flow channels are longitudinally offset from those for the fuel gas flow channels by at least a distance equal to the longitudinal extension of a coupling (requiring that the couplings for one or the other set be slightly inset from a respective longitudinal end of a stratum), this potential problem is avoided.
In the context of separator plate coupling cavity design, it will be apparent from the foregoing that as embossments on one side of the plate constitute depressions on the other side of the plate and vice versa, if one looks merely at one side of such a separator plate, one will see depressions for interconnecting flow channels for one of the reactant gases formed on that side of the separator plate. But with a thin enough plate, one will also see embossments for the other of the reactant gases (these embossments defining depressions on the other side of the separator plate for interconnecting flow channels for the other reactant gas that flows on the other side of the separator plate).
The length in the axial sense of any given depression will be expected to be at least roughly equal to the distance between adjoining apices of the of the MEA layer in order that the pressure drop, as the reactant gas flows form one flow channel through the depression to the adjacent flow channel, will not be unduly large. The dimensions overall of the depression, and the shaping of the depression, can be selected empirically to give the requisite pressure drop, and to provide the desirable turbulence within the flowpath arrangement.
The invention enables ready creation and economical manufacture of suitable structure for providing flow fields without the need for any external manifolding other than for flowpath end connections. No external flowpath control elements are required for the implementation of the invention. Economy of manufacture of internal configuration can be met, while also meeting the usual objectives of space efficiency and usual flowpath desiderata such as supply of reactant gas to a relatively high proportion of the accessible active surface area of the adjacent MEA layer, and long length of flowpath relative to cross-section. (The use of small external plena may, however, facilitate supply and exhaust of reactant gases to and from the flow fields.) Note in particular that once the stack space required for the flow channels is established, the channel interconnection arrangement of the invention can be accomplished without requiring any additional space, and equally, no extra components (except possibly flow channel end stoppers, and even these can be omitted if suitable end crimping or sealing is used) need be added to contribute to stack weight, thereby contributing to both the volumetric and weight power density efficiency of the stack. The invention also facilitates the ready design of uniform multiple flowpaths in a given reactant gas flow field.
It is also an aspect of the invention to use hydrogen in a fuel cell stack made up of fuel cells having separator plates as heretofore described and connectable via an anode terminal and a cathode terminal to an external load. Each fuel cell has an MEA layer and two discrete associated reactant-gas impermeable separator layers. The MEA layer has a porous anode electrode, a porous cathode electrode, an electrolytic membrane layer disposed between the two electrodes, an anode electro-catalyst layer disposed between the electrolytic membrane layer and the anode electrode, and a cathode electro-catalyst layer disposed between the electrolytic membrane layer and the cathode electrode. One side of one separator layer in conjunction with the MEA layer provides at least one flowpath of a flow field for hydrogen and one side of the other separator layer in conjunction with the MEA layer provides at least one flowpath of a flow field for a selected oxidant. The flowpaths are constituted over their greater length by parallel transversely spaced and longitudinally extending flow channels interconnected in the vicinity of their ends to form the flowpaths. The MEA layer is installed in the stack between the associated separator layers so that the side of the separator layer that in conjunction with the MEA layer provides flow channels of a flow field for hydrogen faces and is in contact with the anode side of the MEA layer, whilst the side of the separator layer providing flow channels of a flow field for oxidant faces and is in contact with the cathode side of the MEA layer, so that the hydrogen flow channels are closed to form a conduit for supplying hydrogen to the MEA layer and the oxidant flow channels are closed to form a conduit for supplying oxidant to the MEA layer. The fuel cells are stacked in sequence, the anode electrode of the fuel cell at one extremity of the stack being electrically connected to the anode terminal, the cathode electrode of the fuel cell at the other extremity of the stack being electrically connected to the cathode terminal, and the anode electrode of each of the other fuel cells in the stack being electrically connected to the cathode electrode of the next adjacent fuel cell. When the anode terminal and cathode terminal are electrically connected through an external load and for each fuel cell hydrogen is supplied to the hydrogen conduit and oxygen is supplied to the oxidant conduit, then in each fuel cell hydrogen moves from the hydrogen flow field through the anode electrode and is ionized at the anode electro-catalyst layer to yield electrons and hydrogen ions, the hydrogen ions migrate through the electrolytic membrane layer to react with oxygen that has moved from the oxidant flow field through the cathode to the cathode electro-catalyst layer and with electrons that have moved from the anode electrode electrically connected to the cathode electrode, thereby to form water as a reaction product, and a useful current of electrons is thereby produced through the load.